1) Don't recast the characters he helped make iconic. I mean seriously, the voice cast on The Simpsons is as iconic as anything else about the show, and it's been the same since the very beginning. You could do mimics, or change the voices of all Shearer's character entirely...The better, the best option is to retire those characters. The Simpsons is known for its sprawling cast. It hasn't added many new characters in years, and even if it doesn't now, it could easily survive a trimming. Part of the reason so many fans think, have thought for years that the show has been stale, stagnant, is that nothing has changed for years. Every time something does change, it comes off as a publicity stunt. Think of this as an opportunity to prove everyone wrong, once and for all. I mean, South Park survived losing Chef (even though he was probably the best thing about it). Shearer doesn't even voice any of the main characters. And The Simpsons already survived losing its best voice actor, Phil Hartman, and retired his characters, right? If worst come to worst, simply bring on Kelsey Grammer full-time. That would be a challenge. But it could also be the Sylar-ing of Sideshow Bob. I think I could live with that, though.
2) This is a sign that The Simpsons is finally winding down. Finally losing one of the iconic voice actors, after incredibly keeping all of them for twenty-six seasons, is already historic. But if they've lost one, chances are becoming better that they will lose more. And as I've already suggested, The Simpsons is nothing if not its voice cast plus everything else. Emphasis on voice cast. The show will end. I'm not saying this because I'm gleeful about the prospect. But it is inevitable. And now we have a better idea of how it will happen. And it seems pretty obvious now, doesn't it?